5
LT1944
Choosing an Inductor
Several recommended inductors that work well with the
LT1944 are listed in Table 1, although there are many other
manufacturers and devices that can be used. Consult each
manufacturer for more detailed information and for their
entire selection of related parts. Many different sizes and
shapes are available. Use the equations and recommenda-
tions in the next few sections to find the correct inductance
value for your design.
Table 1. Recommended Inductors
PART VALUE (
µ
H) MAX DCR (
Ω
) VENDOR
LQH3C4R7 4.7 0.26 Murata
LQH3C100 10 0.30 (714) 852-2001
LQH3C220 22 0.92 www.murata.com
CD43-4R7 4.7 0.11 Sumida
CD43-100 10 0.18 (847) 956-0666
CDRH4D18-4R7 4.7 0.16 www.sumida.com
CDRH4D18-100 10 0.20
DO1608-472 4.7 0.09 Coilcraft
DO1608-103 10 0.16 (847) 639-6400
DO1608-223 22 0.37 www.coilcraft.com
Inductor Selection—Boost Regulator
The formula below calculates the appropriate inductor
value to be used for a boost regulator using the LT1944 (or
at least provides a good starting point). This value pro-
vides a good tradeoff in inductor size and system perfor-
mance. Pick a standard inductor close to this value. A
larger value can be used to slightly increase the available
output current, but limit it to around twice the value
calculated below, as too large of an inductance will in-
crease the output voltage ripple without providing much
additional output current. A smaller value can be used
(especially for systems with output voltages greater than
12V) to give a smaller physical size. Inductance can be
calculated as:
L
VV V
I
t
OUT
IN MIN
D
LIM
OFF
=
−+
()
where V
D
= 0.4V (Schottky diode voltage), I
LIM
= 350mA
and t
OFF
= 400ns; for designs with varying V
IN
such as
battery powered applications, use the minimum V
IN
value
in the above equation. For most systems with output
APPLICATIO S I FOR ATIO
WUU
U
voltages below 7V, a 4.7µH inductor is the best choice,
even though the equation above might specify a smaller
value. This is due to the inductor current overshoot that
occurs when very small inductor values are used (see
Current Limit Overshoot section).
For higher output voltages, the formula above will give
large inductance values. For a 2V to 20V converter (typical
LCD Bias application), a 21µH inductor is called for with
the above equation, but a 10µH inductor could be used
without excessive reduction in maximum output current.
Inductor Selection—SEPIC Regulator
The formula below calculates the approximate inductor
value to be used for a SEPIC regulator using the LT1944.
As for the boost inductor selection, a larger or smaller
value can be used.
L
VV
I
t
OUT D
LIM
OFF
=
+
2
Current Limit Overshoot
For the constant off-time control scheme of the LT1944,
the power switch is turned off only after the 350mA current
limit is reached. There is a 100ns delay between the time
when the current limit is reached and when the switch
actually turns off. During this delay, the inductor current
exceeds the current limit by a small amount. The peak
inductor current can be calculated by:
II
VV
L
ns
PEAK LIM
IN MAX SAT
=+
−
()
100
Where V
SAT
= 0.25V (switch saturation voltage). The
current overshoot will be most evident for systems with
high input voltages and for systems where smaller induc-
tor values are used. This overshoot can be beneficial as it
helps increase the amount of available output current for
smaller inductor values. This will be the peak current seen
by the inductor (and the diode) during normal operation.
For designs using small inductance values (especially at
input voltages greater than 5V), the current limit over-
shoot can be quite high. Although it is internally current
